1. Field of the Invention
The present invention relates to an anti-friction bearing and, more particularly, to an anti-friction bearing which is suitable for a rotary support section of precision apparatus and equipment such as a computer peripheral device, a video tape recorder (VTR), a CD-ROM, and a laser beam printer.
2. Description of the Prior Art
Anti-friction bearings for a precision apparatus are used in, for example, a hard disk drive device (HDD) which is a computer peripheral device that enables a magnetic disk to be rotated at a high speed by means of a spindle motor. Anti-friction bearings are incorporated in a rotary support section of a spindle motor in a small hard disk drive device of 3.5 inches or smaller. For small hard disk drives anti-friction bearings called miniature bearings are used having a dimension of about 4 mm in inner diameter ×8 mm in outer diameter to 6 mm in inner diameter ×15 mm in outer diameter.
In recent years, the trend has been for the reduction in size of, and the increase in density of, a hard disk drive device. This has been especially true as the increase in density of a hard disk device of 3.5 inches or smaller has been rapidly progressing. In addition, with respect to a 2.5 inch hard disk drive device, such as those mounted in a notebook type personal computer for portable use, both line recording density and track density are high since a recording capacity similar to that of a large 3.5 inch hard disk device is required in spite of the fact that the 2.5 inch hard disk drive device is made thinner and the magnetic disk thereof is smaller in diameter. A high track density of 10 KTPI to 14 KTPI (TPI: track per inch) has been currently achieved in such devices. In addition, the magnetic disk component of these hard disk drives must rotate at higher revolutions in order to increase the data transmission rate. In 3.5 inch hard disk drive device, the magnetic disk revolves at high rates such as 5,400 to 7,200 rpm, and depending on the machine type, can exceed 10,000 rpm.
Because of these increased performance demands, hard disk drive manufacturers have been demanding a bearing, with improved vibration and acoustic characteristics when incorporated in spindle motors used in hard disk drives which operate at a high rate of revolutions. Bearings which are incorporated in spindle motors for hard disk drives are exposed to conditions under which the raceway grooves of the inner and outer rings and the rolling elements are repeatedly subjected locally to stress. Accordingly, surface roughness develops due to the rolling fatigue of the raceway groove faces and the surfaces of the rolling elements, resulting in an increase in vibration and deterioration of the acoustic characteristics and furthermore, shortening of the life of a bearing. Because of this, the inner and outer rings and rolling elements of a bearing are generally made of a high carbon chromium bearing steel (JIS-SUJ2 or one which corresponds to JIS-SUJ2) which has been quenched to achieve high hardness of HRC 58 to 64 as to the surface hardness in order to prevent the raceway groove faces and the faces of the rolling elements from being worn and/or damaged. However, since high carbon chromium bearing steel is poor in anti-corrosion property, the inner and outer rings are made of martensitic stainless steel (corresponding to JIS-SUS 440 C) in cases in which an anti-corrosion property is required.
Stainless steel has good anti-corrosion properties, however, when molten stainless steel is being solidified, Cr (chromium) in the steel is bound with C (carbon), thereby forming eutectic carbides, which tend to become large in crystal size.
When these large carbides exists on the rolling face of a bearing or the surface of a rolling element, it is difficult to obtain a high precision machine-finished surface due to the difference in hardness and cutting properties between these carbides and the metal matrix surrounding it, thereby reducing the quietness from the initial use of bearing.
The lack of precision of the machine-finished surface due to carbides can be corrected or rectified by additional finishing, grinding and cutting steps. Additional processing steps of grinding and cutting of a hard carbide, however, are not desirable because this leads to an increase in manufacturing cost.
Furthermore, eutectic carbide particles and the surrounding metal matrix have different anti-wearing properties, which may cause part of a eutectic carbide particle to brake off from the surface of the metal matrix during the rotation of a bearing. This difference in anti-wearing properties which can cause part of the eutectic carbide to brake off may disturb the surface configuration (surface roughness) of a bearing, becoming a cause for vibration of a bearing, thereby markedly reducing the quietness. Because of this, it is desired to reduce the size of the carbide particles as much as possible, and to prevent it from being unevenly distributed, in order to maintain the quietness of a bearing.
As for literature on conventional technology concerning stainless steel for which the size of these carbide particles are made small, mention may be made of Japanese Patent Gazette, Patent Publication No. Hei 5/1993-2734. This publication discloses an anti-friction bearing made of stainless steel consisting of 0.6 to 0.75% by weight of C, 0.1 to 0.8% by weight of Si, 0.3 to 0.8% by weight of Mn, 10.5 to 13.5% by weight of Cr, with the remainder being Fe and unavoidably mixed impurity/impurities, and eutectic carbides contained therein being 20 μm or less in the major axis and 10% or less in area ratio. It also discloses an anti-friction bearing made of stainless steel consisting of 0.6 to 0.75% by weight of C, 0.1 to 0.8% by weight of Si, 0.3 to 0.8% by weight of Mn, 10.5 to 13.5% by weight of Cr, with the remainder being Fe and unavoidably mixed impurity/impurities, and eutectic carbides contained therein being 10 μm or less in the major axis and 5% or less in area ratio.
In addition to carbide particles, other particles which are high in hardness may be formed in stainless steel, such as non-metallic inclusions which are formed through the binding of impurities in the raw material and gas dissolved in molten steel (such as nitrogen and oxygen). Ti (titanium) which is present as an impurity in the raw material, forms a non-metallic inclusion in the form of TiN (titanium nitride). TiN is high in hardness in comparison with the surrounding metal matrix, causing a similar problem to that caused by the above-mentioned carbide.
Additionally, with stainless steel, Si (silicon) and Al (aluminum), etc. are added as deoxidizing agents in order to remove oxygen dissolved in molten steel. Oxide type non-metallic inclusions such as SiO2 and Al2O3 tend to be created by the bonding of oxygen with Si and Al. Since such oxide type non-metallic inclusions such as SiO2 and Al2O3 are high in hardness in comparison with the surrounding metal matrix, a similar problem to that caused by the above-mentioned carbide may be caused. The above-mentioned Japanese Patent Gazette, Patent Publication No. Hei 5/1993-2734 discloses stainless steel for which the size of carbide particles is made as small as possible, however, no attention nor any consideration is given to non-metallic inclusions such as TiN, SiO2 and Al2O3, and thus there exists a problem in achieving a finished surface with a high degree of precision.
As for literature regarding conventional technology concerning stainless steel for which the contents of Ti (titanium) and O (oxygen) which are elements which create such non-metallic inclusions, are reduced, mention may be made of U.S. Pat. No. 5,030,017.
U.S. Pat. No. 5,030,017 (the '017 patent) discloses an anti-friction bearing made of a bearing material obtained by applying a carbonizing heat treatment or a carbonizing-nitriding heat treatment to stainless steel for which the contents of Ti (titanium) and O (oxygen), which are elements which create non-metallic inclusions, are reduced to 40 ppm or less and 12 ppm or less respectively.
In the '017 patent, the upper limits of the Ti (titanium) and O (oxygen) contents are set forth and defined, however, the '017 patent relates to an anti-friction bearing whose field of utilization includes automobile, agricultural machinery and equipment, construction machinery and equipment, iron and steel machinery and equipment, etc., i.e. where the size of the bearing is greater than for a precision instrument such as a HDD. The bearing in the '017 patent is characterized in that the retained austenite quantity in the surface layer of the bearing is 10 to 25% by volume, and especially an anti-friction bearing of a long life for use in a transmission and an engine.
In high precision machines, especially a hard disk drive device, it is well known in the art that there are strict requirements on the vibration and acoustic characteristics. The stainless steel disclosed in U.S. Pat. No. 5,030,017 is not suitable as a material to be used in a bearing for use in rotary support section of a hard disk drive device because of its vibration and acoustic characteristics.
U.S. Pat. No. 5,639,168 discloses a ball bearing in which the anti-load property and anti-shock property have been improved by limiting the quantity of the retained austenite contained in the bearing steel or stainless steel constituting the raceway groove surface of a ball bearing to 2% by volume or less.
However, the ball bearing disclosed in the U.S. Pat. No. 5,639,168 does not pay any attention nor take into account any non-metallic inclusion such as TiN, SiO2 and Al2O3, and thus there exists a problem in achieving a finished surface of a high degree of precision of the raceway groove surface of an anti-friction bearing, and in improving the quietness thereof. The ball bearing of U.S. Pat. No. 5,639,168 does not limit the amount of Ti or O.